The present invention is directed to continuous sheet processing such as is found in the papermaking industry, and more particularly to a device and method for calibrating a non-destructive sheet strength measuring system adapted for such continuous processing.
Non-destructive methods and devices for determining the strength of a continuously moving sheet are disclosed in commonly-owned U.S. applications Ser. No. 730,406, filed May 2, 1985 and Ser. No. 920,107, filed Oct. 16, 1986, and in U.S. Pat. No. b.varies.,864,851, issued Sept. 12, 1989 and U.S. Pat. No. 4,866,984, issued Sept. 19, 1989. These applications and patents are incorporated herein by reference. Using the methods and devices disclosed in these applications and patents, the burst strength of a sheet can be computed without damaging the sheet. The term "burst strength" is used herein to denote both a uniformly directed (omni-directional) burst such as occurs, for example, in the well known Mullen test for determining paper strength and a uni-directed burst such as would occur in a tensile test wherein a sheet sample is pulled apart in a specific direction relative to its length.
The methods disclosed in the cited applications and patents, and the devices for performing these methods, rely upon the relationship between the burst strength of a sheet (e.g. a paper sheet) and the force required to deflect the sheet into an unsupported region. In brief, the methods disclosed in these applications comprise the steps of: supporting the moving sheet around an unsupported region (e.g. with an annular support); nondestructively deflecting the sheet into the unsupported region; measuring the amount of force exerted to cause the deflection; and calculating the burst strength of the moving sheet from the measured deflection force using empirically determined mathmatical formulas which relate deflection force to sheet burst strength.
Preferably, the moving sheet is deflected by a constant distance into the unsupported region and is moved by rollers which span the width of the sheet and hold the sheet under a constant tensile force. However, if the tensile force on the moving sheet and the amount of deflection cannot be held constant, then the value of the sheet strength which is calculated based upon the deflection force may be further refined by incorporating into the strength calculation one or more variables indicative of the changing tensile force and the changing distance that the sheet is deflected into the unsupported region. Values indicative of the sheet velocity, basis weight and thickness may also be utilized in the strength calculations. Sheet strength equations which incorporate values indicative of these parameters are disclosed in the previously cited applications.
FIG. 1A is a cross-sectional view of one portion of a non-destructive strength measuring system 10A which is disclosed in U.S. Pat. No. 4,864,851. In this system, the upper and lower sensor supports, 12 and 14, are provided over-lapping opposed sides of a moving sheet 11. The annular sheet support (or "ring") 16 is disposed on the lower sensor support 14 with an upper surface 16a of the ring 16 contacting the moving sheet 11 to support the sheet in an annular fashion. A rotatable sensor wheel 18 is aligned with the hole in the center of the ring 16. The sensor wheel 18 is brought into contact with the upper surface of the sheet 11 to deflect the unsupported portion of the moving sheet 11 below the top surface 16a of the ring 16. The wheel 18 is rotated by frictional contact with the moving sheet 11 so that the wheel 18 moves at the same velocity as the sheet 11 and thus does not to abrade the top surface of the sheet 11. A piezoelectric force sensor 20 and a radially displaceable button 21 are provided at the periphery of the wheel 18 for determining the amount of force exerted between the sheet 11 and sensor wheel 18 when the sheet 11 is deflected into the center of the ring 16. A slip ring coupler 22 connects the sensor 20 to an electronic computer to supply the computer with force measurement signals, F, from the piezoelectric force sensor 20.
An air actuated piston 23 determines the distance that the wheel 18 deflects the sheet 11 below the upper ring surface 16a. For added accuracy, a distance measuring sensor 24, which may, for example, be a magnetic reluctance gap detector, may be provided on the lower support 14 to precisely measure the amount of deflection. The deflection distance is sent to the computer as a vertical deflection distance signal Z. The signals, F and Z, from sensors 20 and 24, respectively, are received by the computer and used to calculate the sheet burst strength using the empirically determined formulas disclosed in the cross-referenced applications and patents.
The empirical formulas include certain fixed values that are set prior to a strength measurement run and other run-time variable parameters, including values representing the measured amount of deflection and deflection force. For example, U.S. Pat. No. 4,864,851 discloses the following paper sheet strength equation for use with the strength measuring system 10A when it is positioned downstream of the calender stack in a papermaking machine: ##EQU1## where
S is the computed sheet strength which correlates with the Mullen strength,
L is the instantaneous deflection force as measured by the piezoelectric force sensor 20,
L.sub.AV is the average deflection force across the width of the sheet 11 as measured by the piezoelectric force sensor 20,
W is the basis weight of the sheet 11,
T is the thickness of the sheet 11,
V is the velocity of the paper leaving the last calender roll of the paper making machine,
f(Z) is a function of the deflection distance, Z, and
A,B,C,D,E and F are constants.
The values of the constants may be experimentally determined using well known equation curve fitting techniques. Similarly, the function f(Z) can also be experimentally determined by taking deflection force measurements of a sheet having known constant basis weight, thickness, velocity and strength, and solving the equation (1) for f(Z).
FIG. 1B shows a cross sectional view of another continuous-run non-destructive strength measuring system 10B which is disclosed in the above-referenced U.S. Pat No. 4,866,984. Like elements are referenced by numbers corresponding to those of FIG. 1A and need not be described again. This strength measuring system operates in a manner similar to the system of FIG. 1A. However, the support ring of this particular embodiment is quadrally divided into segments 16A, 16B, 16C and 16D (only two shown) with each segment being suspended by a spring 21 having a predetermined suspension force. A corresponding set of force measuring sensors 20A-20D (two shown) are coupled to the respective ring segments 16A-16D for developing respective deflection force signals L.sub.a, L.sub.b, L.sub.c, and L.sub.d (only two illustrated) of which, one pair, L.sub.a and L.sub.c, indicate machine direction force parameters and a second pair, L.sub.b and L.sub.d, indicate cross-direction force parameters. The L.sub.a and L.sub.c force signals are generated by the force sensors 20A and 20C associated with the ring segments disposed on opposite sides of the ring in the machine direction, as illustrated in FIG. 1B. The remaining two force signals, L.sub.b and L.sub.d, are generated by the force sensors 20B and 20D associated with the remaining two opposing ring segments. It is to be noted that the slip ring coupler 22 and wheel mounted sensor 20 of FIG. 1A are not required for the sensor wheel 18' of FIG. 1B.
Using the strength sensor system of FIG. 1B, the sheet strength can be non-destructively determined in both the machine direction and the cross direction. For example, U.S. patent application Ser. No. 920,107 discloses the following sheet strength equations for calculating sheet strength in the machine direction and the cross direction: ##EQU2## where S.sub.md and S.sub.cd are, respectively, the machine and cross direction sheet strengths. T.sub.n is the average sheet tension measured across the entire width of the sheet, Z is the amount of sheet deflection and A,B,C,D, E,F,G,H, and J are constants. As in equation (1) above, the constants are empirically determined using well known equation curve fitting techniques. Z may be determined using the previously mentioned magnetic reluctance gap detector. Techniques and devices for measuring sheet tension are also known. The constant E need not necessarily have the same value in both equations (2) and (3).
U.S. patent application Ser. No. 920,107 also discloses that the Mullen strength may be computed with the following equation which combines the outputs from all four of the split ring force sensors: ##EQU3## where S.sub.mu is the Mullen strength and the other parameters are the same as previously disclosed. The constants, however, are not necessarily the same values in each of equations 1-4.
The non-destructive sheet strength measuring sensors of FIGS. 1A and 1B may be scanned back and forth across the width of sheet as the sheet is being continuously produced by the sheet forming machine. The signals from these sheet strength sensors may then be sent to a computer which determines the strength profile of the sheet in both the machine and cross directions. The strength profile can then be displayed to the operator either in graphical form or numerically. Based upon the displayed profile, the operator may adjust the operation of the sheet forming machine to increase or decrease the overall sheet strength or to adjust the sheet strength in one or more localized areas across the width of the sheet. For example, in a paper mill, the mill operator can adjust the sheet strength by increasing or decreasing the degree to which the pulp is refined, by changing the jet-to-wire ratio or by altering the volume of the flow of pulp from the headbox at one or more particular areas across the width of the sheet. Other means for adjusting the overall strength of paper sheet and the sheet strength at localized areas are known in the art.
While the non-destructive strength measuring systems 10A and 10B of FIGS. 1A and 1B overcome many problems associated with previously known destructive methods for determining the burst strength of a sheet 11, there still remained a problem of how to calibrate such systems relative to industry accepted standards based upon destructive strength testing. To complicate matters further, in some circumstances it may be necessary to calibrate the systems relatively frequently. For example, in papermaking, the relationship between the burst strength of the paper sheet and the force required to deflect the sheet can change as a result of using different types of trees for the raw material, as a result of wood fiber length changes, changes in lignin content of the wood and so forth.
Prior to the present invention, sheet samples had to be periodically obtained from the continuous sheet production line and the results of destructive laboratory strength tests of these samples used to calibrate the non-destructive sheet strength measuring systems. The samples were cut from a sheet and sent to the laboratory for destructive testing. The samples were then placed in a standard stationary testing instrument where, for example, the burst strength of the paper sheet was determined by industry accepted methods such as the destructive Mullen burst pressure test and/or tensile strength testing in the machine direction and cross direction. The non-destructive strength measuring system (e.g. 10A or 10B) was then calibrated with the strength values obtained from these industry accepted stationary standard destructive tests. Such calibrations were performed by altering the values of the constants in the strength equations so that the strength values obtained from the non-destructive strength measuring systems agreed with the laboratory strength measurements.
The non-destructive sheet strength measuring systems of the previously referenced co-pending applications represent a great advance over the prior art since they permit continuous monitoring of the sheet strength, without damage to most of the sheet. Nevertheless, the process of periodically obtaining samples from the production line and bringing the samples to the laboratory for destructive testing to calibrate the non-destructive strength measuring systems can be a source of undesirable delay. Specifically, relatively long periods of time may pass while the laboratory tests are performed before it is discovered that the non-destructive sheet strength measuring system is out of calibration. In the corresponding interim, a large amount of sheet material can be produced. However, the strength of such sheet material will be questionable because of the non-calibrated condition of the non-destructive strength measuring system. Thus, there is a need for the present calibration system which can periodically calibrate the strength measuring systems of the referenced applications automatically and with minimal delay.